Our present invention relates to a polarization transformer used in the microwave field. The interposition of such a transformer in the path of microwaves serves to convert the incident wave transmitted with a given polarization into an outgoing wave of different polarization. More particularly, such a transformer converts a wave having a linear polarization into a wave having a circular polarization, and vice versa, wherever such transformation is desired.
In the electromagnetic-detection field, for example, it is possible to change from the linear polarization of the incident wave to a circular polarization of the outgoing wave if it is desired to eliminate rain echoes, and in case of interference one may reduce the power of the interfering radiation by inverting this circular polarization. In the case where the objects pursued have for example an equivalent surface weak in circular polarization, their detection is facilitated by changing to a linear polarization. Different types of polarization transformers exist which are inserted in a free space in the path of a microwave beam or of semiguided waves, e.g. within a horn.
These types of polarization transformers, however, have to be adjusted mechanically when in operation.
A known polarization transformer of this mechanically adjustable type comprises a grating placed substantially in a phase plane and formed by metal strips which are perpendicular to the direction of propagation and include in a first position an angle with the electric-field vector of the radiated wave, e.g. of 45.degree., to transform the linear polarization of the incident wave into a circular polarization. If this polarization transformer is turned about an axis perpendicular to its plane so that the angle between the electric field of the radiated wave and the direction of the metal strips is made equal to 90.degree., the polarization of the incident wave, which is assumed to be linear, remains unaltered. Thus it is possible, by subjecting such a transformer to a rotation effected mechanically, to vary the polarization of the transmitted wave and convert in the described example the circular polarization into a rectilinear polarization.
Another conventional polarization transformer, also controlled mechanically, comprises a network of conductive wires mounted on thin dielectric supports. The wires are disposed in a plane perpendicular to the direction of propagation and include with the electric-field vector an angle of 45.degree., for example, in a first position corresponding to the creation of a circular polarization at the output of the polarizer. A rotation of this system in the aforementioned manner permits avoiding an alteration of the polarization of the incident wave, assumed to be rectilinear, by orienting the conductive wires perpendicular to the electric-field vector.
A further known polarization transformer comprises networks of conductive wires embedded within a set of dielectric plates whose thickness is such that the capacitive admittance of these networks is equal to half the inductive admittance of the networks of wires contained therein. The rotation of such a unit about an axis perpendicular to its plate faces enables the polarization of the outgoing waves to be changed in the way discussed above.
The physical displacement of at least a part of the transformer, required for the desired rotary adjustment, is often difficult and in some cases even impossible. Mechanical rotation of such a polarization transformer disposed in front of a horn constituting a primary source of a radar, for instance, may be prevented by the presence of dipoles placed around the horn.